The invention relates to an arrangement ensuring the automatic correction of the relative positions of data signals transmitted via two channels through which the same digital information is conveyed and more particularly, to diversity data transmission in which, because of unpredictable variations in the channel paths, either channel may have a relative phase delay which may amount to a few bits. After correction, a switch may be commanded from the lesser-quality channel to the higher-quality channel. Each of these channels is connected to the write input of a buffer store and the write clocks for each of said channels is the input clock of each channel. The read clock of each buffer store is aligned with the aid of a phase locked loop on the write clock of the higher-quality channel.
Such an arrangement can be used each time the same digital information is transmitted through two different channels in order to ensure an improved reliability of the transmission.
On transmission, a coupler or a separator routes the information to the channels. On reception, external evaluation criteria are applied to a logic circuit which determines the choice of the higher-quality channel; the problem then arising is how to effect a switch from one channel to the other without loss of information.
This arrangement is used more specifically for transmission via microwave links in diversity-reception systems, which are based on the hypothesis that the probability that two transmission channels fail simultaneously is very low. The sole auxiliary channel, common to n normal channels (n usually being equal to 7), conveys the information of the channel which degrades at a given instant. When this channel improves again, the auxiliary channel is released, so that it can again be temporarily substituted for another failing channel.
The binary information processing equipment located downstream of the switching arrangement (for example automatic switching networks in the case of telephone transmission) synchronize themselves with the signal itself. But the propagation times of the same information transmitted over two different transmission channels are not absolutely identical; there is a delay difference between the two data streams which depends on the atmospheric conditions and which is larger for greater information rates (from 2 Mbits/s to 140 Mbits/s). This delay difference may be sufficient to introduce, on reception, an additional transition in the clock signal and a shift between the respective information streams transmitted over the two channels which may fluctuate by several bits. At the instant the switch from one channel to the other is effected, the message is truncated and one portion is transmitted over the first channel and an other portion is transmitted over the second channel. When no precautions are taken, a discontinuity occurs in the message due to the fact that bits got lost or were added. The result is that all the material downstream has lost its synchronization. This causes the loss of a considerable number of bits.
In order to obviate these disadvantages several channel switching arrangements have been proposed, for example the arrangement described in European Patent Application No. 0 060 751, which comprises inter alia the buffer memory associated with each channel as mentioned above. In this arrangement, the duration of the transmitted data is prolonged in a ratio of 1:n before the switch from one channel to the other is effected, this switch therefore taking place at the level of fictive data, that is to say data whose duration is n times longer than that of the real data. This results in a comparatively complicated structure for the buffer store. In addition, with respect to the phase shift commands, counting the divergences is performed with the aid of a measuring window obtained by means of a monostable circuit and fixing a predetermined delay for effecting a phase shift of 2.pi./n.